Laser light sources widely used in industrial applications and as devices incorporated into consumer equipment include semiconductor laser diodes and solid state laser light sources and similar. Moreover, wavelength conversion laser light sources are light sources used to obtain laser light at wavelengths difficult to obtain from direct oscillation of semiconductor laser diodes and solid state laser light sources.
In a wavelength conversion laser light source, the laser light frequency, that is, the wavelength, is converted through nonlinear optical effects such as SHG (Second Harmonic Generation), in which light at twice the frequency (the second harmonic) of the fundamental wave laser light (hereafter abbreviated to fundamental wave) incident on a wavelength conversion element is generated, and SFG (Sum Frequency Generation), in which light at the frequency which is the sum of two frequencies (sum frequency) of incident light with two frequencies is generated.
FIG. 23 shows an example of a wavelength conversion laser light source, proposed in the prior art, which generates the second harmonic. The wavelength conversion laser light source comprises a fundamental wave laser light source 111 which generates a fundamental wave, a lens 112 to condense and cause incidence on a wavelength conversion element 113 of the fundamental wave emitted from the fundamental wave laser light source 111, the wavelength conversion element 113 which generates the second harmonic of the fundamental wave, and a dichroic mirror 114 which separates the fundamental wave FL (transmissive fundamental wave laser) and the second harmonic SL (wavelength conversion laser); the fundamental wave is condensed and is passed once through the wavelength conversion element 113 to generate the second harmonic.
The wavelength conversion element 113 comprises a nonlinear optical crystal; it is necessary to control the crystal orientation and the period of the poled structure such that the phases of the fundamental wave and the second harmonic coincide. In particular, a quasi-phase-matched wavelength conversion element using a periodically polarization-inverted structure can perform wavelength conversion with high efficiency, and, depending on the design of the poled period, can convert a fundamental wave of arbitrary wavelength into the second harmonic, and so is widely used.
Here, the efficiency η of wavelength conversion from the fundamental wave to the second harmonic is given by equation (1) below, where L is the interaction length in the wavelength conversion element, P is the fundamental wave power, A is the beam cross-sectional area in the wavelength conversion element, and Δk is the phase difference between the fundamental wave and the second harmonic, relative to the phase matching condition.ηα(L2×P/A)×sinc2(Δk×L)  (1)
From the above equation (1), it is seen that by lengthening the wavelength conversion element interaction length L, highly efficient wavelength conversion can be performed.
However, if the interaction length L is made long, the conditions for making the phase difference Δk between the fundamental wave and the second harmonic small (for example, the fundamental wave incidence angle and wavelength conversion element temperature conditions) become more strict, so that the drop in wavelength conversion efficiency becomes prominent, and for practical purposes the interaction length L is limited. For example, due to wavelength conversion element temperature conditions, the interaction length L has been limited, and it has been difficult to raise the efficiency. The temperature of the wavelength conversion element when the phase difference Δk between the fundamental wave and the second harmonic is 0 is called the phase-matching temperature, and the wavelength conversion element temperature width at which the wavelength conversion efficiency is half is called the temperature tolerance width.
In the past, there have been numerous proposals to improve the wavelength conversion efficiency of a wavelength conversion laser light source. For example, in Patent Document 1, it is proposed that wavelength conversion efficiency be raised by using a plurality of wavelength conversion elements and optical condensing means. And, in Patent Document 2, it is proposed that fundamental wave reflection means be used to provide a fundamental wave reflecting member for a wavelength conversion element, to cause repeated incidence on the wavelength conversion element. Further, in Patent Document 3, it is proposed that a wavelength conversion element be arranged between opposing concave mirrors, and that wavelength conversion of the fundamental wave traveling back and forth be performed.
However, in each of the above configurations proposed in the prior art, the wavelength conversion efficiency of a wavelength conversion laser light source can be improved, but there has been the problem that the wavelength conversion efficiency fluctuates greatly due to temperature changes of the wavelength conversion element.
Patent Document 1: Japanese Patent Application Laid-open No. H11-44897
Patent Document 2: Japanese Patent Application Laid-open No. 2006-208629
Patent Document 3: Japanese Patent Application Laid-open No. 2005-268780